The present invention relates to a generator of at least one reference voltage with improved performance. Reference voltage generators may be used in a great number of applications such as converters in which it is necessary to have a voltage value that is precise and stable whatever the environmental conditions are. This is notably the case when the reference voltage is based on the energy band. These voltage generators are known by the English name of bandgap generators in the literature. In an integrated circuit the potential barrier of a PN junction corresponding to the forbidden bandwidth of the semiconductor, that is 1.205 volts in the case of silicon, is used as a reference voltage.
It is tried for these reference voltage generators to have a temperature gradient that is well known and even often to be independent of temperature variations. These reference voltage generators are constituted by various electronic components which themselves have their own dependence on temperature and the control of the temperature gradient of the assembly is difficult.
The value of the reference voltage delivered by this reference voltage generator must not be dependent on the manufacturing process of the various electronic components of the generator. These reference voltage generators are produced in the form of monolithic integrated circuits and it is a known practice that components having the same characteristics finally have different cost.
Moreover, it is tried for the reference voltage delivered by such generators to be the least possible affected by faults of the supply source that feeds them. The signals delivered by the supply sources inevitably comprise disturbances: parasitic random noise, noise, voltage peaks. These faults need not affect the reference voltage delivered by the generator. In conclusion, it is tried for the reference voltage generator to have as large a power supply rejection ratio as possible on a large frequency band. It is the ratio between a variation of the output voltage of the reference voltage generator brought about by a variation of the supply voltage and said variation of the supply voltage, this magnitude is known by the English abbreviation PSRR for Power Supply Rejection Ratio.
Finally, it is also tried for the reference voltage generator to have a good load rejection and to have the shortest possible response time at the start.
The reference voltage generators of known type are such that their output voltage combines with appropriate weight factors a base-emitter voltage of a bipolar transistor and a voltage proportional to the absolute temperature T. The choice of the weight factors is made so that the voltage variations proportional to the absolute temperature compensate for those of the base-emitter voltage of the bipolar transistor.
An example of a reference voltage generator known from the article xe2x80x9cA Simple Three-Terminal IC Bandgap Referencexe2x80x9d, A. Paul BROKAW, IEEE Journal of Solid State Circuits, vol. SC-9, no. 6, December 1974, pp. 388 to 393, is illustrated in FIG. 1. It is formed by an input stage 1 having two branches 10, 11 connected between two supply terminals 20, 21, one terminal 20 connected to a high potential Vcc, the other terminal 21 connected to a low potential Vee, generally ground. In each of the branches 10, 11 is found at least a bipolar transistor Q1, Q2 and these transistors do not have the same size of emitter. This input circuit 1 combines a base-emitter voltage of one of the bipolar transistors Q2 with a voltage proportional to the absolute temperature (known by the voltage name PTAT, PTAT being the English abbreviation for Proportional To Absolute Temperature) and it is the voltage resulting from this combination that forms the reference voltage Vref.
This input circuit 1 is associated with an operational amplifier 2 which, while attenuating the variations of the supply voltage Vccxe2x88x92Vee, maintains the same current in the two branches 10, 11. The operational amplifier is configured to have a largest possible gain.
More precisely, the two transistors Q1, Q2 have a common base, their collectors connected to the supply terminal 20 connected to the potential Vcc via a resistor R2, R3, respectively. The emitter of the first transistor Q1 is connected to the other supply terminal 21 by a series combination 12 of two resistors R1, R0. The emitter of the second transistor Q2 is connected to the other supply terminal 21 via one of the resistors R0 of the series combination 12. It is supposed that the emitter surface of the first transistor Q1 is equal to n (n being an integer greater than one) times that of the second transistor Q2. For example, n may be equal to 8.
The operational amplifier 2 may adopt a conventional form with a differential amplifier stage 13 and an output stage 14. In FIG. 1 the differential amplifier stage 13 comprises a differential pair 15 of transistors Q3, Q4 whose bases form the two differential inputs. The base of the transistor Q3 is connected to the branch 11 at the transistor collector Q2, the base of the transistor Q4 is connected to the branch 10 at the collector of the transistor Q1. The emitters of the transistors Q3 and Q4 are interconnected. They are connected to the supply terminal 21 connected to the potential Vee via a source resistor R4. The collectors of the two transistors Q3, Q4 are each connected to the supply terminal 20 connected to the potential Vcc via a load resistor R5, R6, respectively. The output stage 14 comprises a follower circuit 22 with a transistor Q5 whose emitter is connected to the supply terminal 21 connected to the potential Vee via a resistor R7 whose collector is connected to the supply terminal 20 connected to potential Vcc and whose base is connected to the emitter of the transistor Q4 of the differential amplifier 13.
The output of the reference voltage generator is found at the bases of the transistors Q1, Q2 of the input stage 1 which are connected to the emitter of the transistor Q5 of the output stage 14. The operational amplifier 2 compares the currents flowing in the two branches 10, 11 and provides that they remain substantially equal whatever the variations of the supply power.
The voltage Vref delivered by this reference voltage generator has the value of: Vref=Vbe(Q2)+R0.I0, Vbe(Q2) representing the base-emitter voltage of the transistor Q2 and I0 being the current flowing in the resistor R0.
It may be stated that Vbe(Q2)xe2x88x92Vbe(Q1)=R1.I1.
But Vbe(Q2)xe2x88x92Vbe(Q1)=VT.Log(n) with VT being the thermal voltage. This thermal voltage VT is equal to kT/Q where k is the Boltzmann constant, T the temperature in degrees Kelvin and Q the charge of the electron.
The voltage at the terminals of the resistor R0 is equal to: 2.VT.Log(n).R1/R0 since the same currents are flowing in the transistors Q1, Q2.
The reference voltage Vref is such that:
Vref=Vbe(Q2)+2.VT.Log(n).R1/R0.
The ratio of the resistances R1/R0 may thus be adjusted so that in the sum the variations of the term proportional to VT practically compensate for those of Vbe(Q2). But in an open loop arrangement the reference voltage Vref follows the variations of the supply voltage.
One of the drawbacks of this generator is that the precision of the voltage obtained is not very good if not a high gain operational amplifier is used. But a high gain amplifier has a high energy consumption and needs to be stabilized. Its passband is small and so is its supply voltage rejection.
Another drawback is that the reference voltage generator needs to have a start circuit (not shown). Actually, the circuit is found in a stable mode when no current is flowing in the transistors Q1, Q2 and when they are in a blocked state. The start circuit has for its function to inject a current in the charging circuit of the differential pair thus increasing the emitter voltage of the transistors of the differential pair and, in consequence, the voltage at the base of the transistors of the input circuit. Such a start circuit requires a number of active components, for example, various MOS transistors which operate as switches, a current mirror with bipolar transistors and several resistors. This notably increases the cost of the reference voltage generator.
It is an object of the present invention to propose a reference voltage generator which is as insensitive as possible to supply voltage variations and the manufacturing process, whose dependence on temperature is given and which does not have the disadvantages of the reference voltage generator of FIG. 1, that is, the necessity to utilize a high-gain operational amplifier and the necessity to include a startup circuit.
To achieve this, the present invention relates to a generator of at least one reference voltage comprising, connected between two power supply terminals,
an input stage having a portion that is proportional to the absolute temperature and delivering a potential that is substantially independent of temperature,
an operational amplifier comprising:
a differential amplifier stage connected to the input stage including a charging circuit and a source circuit and
an output stage connected at a first node to the charging circuit, intended to be connected to the input stage by a loop which is closed and delivering the reference voltage.
The source circuit and the charging circuit comprise regulation means for regulating the reference voltage even when the loop connecting the input stage to the output stage is open, which reference voltage is then delivered in a manner substantially independent of the manufacturing process of the generator, variations of the supply voltage and has a given dependence on temperature.
The regulation means impose that with an open loop, during a variation of the supply voltage, substantially the same variation is reflected in the source circuit as in the charging circuit in a way that the voltage appearing on the first node is practically independent of the variations of the supply voltage, the voltage in the source circuit being substantially independent of temperature.
The differential amplifier may comprise a differential pair of transistors and the source circuit may comprise a resistor and a diode connected in series, the resistor being connected to the differential transistor pair and the diode to one of the supply terminals, the diode having a temperature gradient so that, even when the loop is open, said gradient compensates for the temperature gradients of the input stage and of the differential amplifier stage in such a way that the voltage on the terminals of the resistor is substantially independent of temperature and manufacturing process.
The charging circuit may comprise a resistor connected between the first node and one of the supply terminals, the ratio between the value of the resistance of the charging circuit and the value of the resistance of the source circuit being adjusted in such a way that, even with an open loop, during a variation of the supply voltage, substantially the same variation is reflected on the source circuit and on the charging circuit, so that the voltage appearing on the first node is practically independent of the variations of the supply voltage.
The operational amplifier may comprise a compensation circuit connected to the first node and to the output stage at a second node with the closed loop, the compensation circuit and the source circuit maintaining on the first node a voltage that substantially compensates for the voltage produced by the output stage, rendering the voltage on the second node substantially independent of temperature and variations of the supply voltage even when the loop is open.
The compensation circuit may comprise a bipolar transistor whose emitter is connected to one of the supply terminals through a resistor, whose collector is connected to the first node and whose base is connected to the output stage at the second node.
The output stage may comprise a follower circuit including a bipolar transistor whose emitter is connected to one of the supply terminals through at least one resistor and to the loop when it is closed, whose collector is connected to the other supply terminal and whose base is connected to the first node, one output of the generator being found at the emitter of the bipolar transistor.
The output stage may comprise a follower circuit with a including transistor whose emitter is connected to one of the supply terminals through a voltage divider bridge and to the loop when it is closed, whose collector is connected to the other supply terminal and whose base is connected to the first node, one output of the generator being found at a common point between two resistors of the voltage divider bridge.
The output stage may comprise, in association with the follower circuit, a regulation circuit for regulating the temperature gradient of the voltage on the first node, this regulation circuit being connected between the first node and one of the supply terminals and being connected to a common point between two resistors of the voltage divider bridge, this regulation circuit generating a current whose temperature gradient is adjustable by the choice of the resistors of the bridge.
The regulation circuit may comprise a bipolar transistor whose emitter is connected to one of the supply terminals through a resistor, whose collector is connected to the first node and whose base is connected to the common point between two resistors of the voltage divider bridge, one output of the generator being found at the emitter of the transistor of the regulation circuit.
The regulation circuit may co-operate with an additional circuit that has a transistor for forming a current mirror, the output being found at the emitter of the transistor of the additional circuit.
It may be interesting in certain applications that the generator comprises a standby circuit for putting the generator in the standby mode, the standby circuit including various pairs of complementary MOS transistors located in the differential amplifier stage and a pair of complementary MOS transistors located in the output stage, these MOS transistors being controlled by a standby mode control device.
This generator is adapted in all respects for delivering a reference voltage based on the forbidden energy band of a semiconductor material.
The invention also relates to a converter including a generator according to the invention and an apparatus intended for the reception and transmission of radio telecommunication signals including a generator according to the invention. Such an apparatus may be, for example, a telephone which may include, for example, a converter according to the invention.
Such converters and radio telecommunication apparatus which may advantageously include a generator according to the invention are described abundantly in the literature with other types of generators.